ext4: fix race when setting bitmap_uptodate flag

In ext4_read_{inode,block}_bitmap() we were setting bitmap_uptodate()
before submitting the buffer for read.  The is bad, since we check
bitmap_uptodate() without locking the buffer, and so if another
process is racing with us, it's possible that they will think the
bitmap is uptodate even though the read has not completed yet,
resulting in inodes and blocks potentially getting allocated more than
once if we get really unlucky.

Addresses-Google-Bug: 2828254

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 44 insertions, 19 deletions

diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
index f9e2cd8cf711..a2cff2b9d5b5 100644
--- a/fs/ext4/balloc.c
+++ b/fs/ext4/balloc.c
@@ -336,10 +336,10 @@ err_out:
  * Return buffer_head on success or NULL in case of failure.
  */
 struct buffer_head *
-ext4_read_block_bitmap(struct super_block *sb, ext4_group_t block_group)
+ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group)
 {
 	struct ext4_group_desc *desc;
-	struct buffer_head *bh = NULL;
+	struct buffer_head *bh;
 	ext4_fsblk_t bitmap_blk;
 
 	desc = ext4_get_group_desc(sb, block_group, NULL);
@@ -348,9 +348,9 @@ ext4_read_block_bitmap(struct super_block *sb, ext4_group_t block_group)
 	bitmap_blk = ext4_block_bitmap(sb, desc);
 	bh = sb_getblk(sb, bitmap_blk);
 	if (unlikely(!bh)) {
-		ext4_error(sb, "Cannot read block bitmap - "
-			    "block_group = %u, block_bitmap = %llu",
-			    block_group, bitmap_blk);
+		ext4_error(sb, "Cannot get buffer for block bitmap - "
+			   "block_group = %u, block_bitmap = %llu",
+			   block_group, bitmap_blk);
 		return NULL;
 	}
 
@@ -382,25 +382,50 @@ ext4_read_block_bitmap(struct super_block *sb, ext4_group_t block_group)
 		return bh;
 	}
 	/*
-	 * submit the buffer_head for read. We can
-	 * safely mark the bitmap as uptodate now.
-	 * We do it here so the bitmap uptodate bit
-	 * get set with buffer lock held.
+	 * submit the buffer_head for reading
 	 */
+	set_buffer_new(bh);
 	trace_ext4_read_block_bitmap_load(sb, block_group);
-	set_bitmap_uptodate(bh);
-	if (bh_submit_read(bh) < 0) {
-		put_bh(bh);
+	bh->b_end_io = ext4_end_bitmap_read;
+	get_bh(bh);
+	submit_bh(READ, bh);
+	return bh;
+}
+
+/* Returns 0 on success, 1 on error */
+int ext4_wait_block_bitmap(struct super_block *sb, ext4_group_t block_group,
+			   struct buffer_head *bh)
+{
+	struct ext4_group_desc *desc;
+
+	if (!buffer_new(bh))
+		return 0;
+	desc = ext4_get_group_desc(sb, block_group, NULL);
+	if (!desc)
+		return 1;
+	wait_on_buffer(bh);
+	if (!buffer_uptodate(bh)) {
 		ext4_error(sb, "Cannot read block bitmap - "
-			    "block_group = %u, block_bitmap = %llu",
-			    block_group, bitmap_blk);
-		return NULL;
+			   "block_group = %u, block_bitmap = %llu",
+			   block_group, bh->b_blocknr);
+		return 1;
 	}
+	clear_buffer_new(bh);
+	/* Panic or remount fs read-only if block bitmap is invalid */
 	ext4_valid_block_bitmap(sb, desc, block_group, bh);
-	/*
-	 * file system mounted not to panic on error,
-	 * continue with corrupt bitmap
-	 */
+	return 0;
+}
+
+struct buffer_head *
+ext4_read_block_bitmap(struct super_block *sb, ext4_group_t block_group)
+{
+	struct buffer_head *bh;
+
+	bh = ext4_read_block_bitmap_nowait(sb, block_group);
+	if (ext4_wait_block_bitmap(sb, block_group, bh)) {
+		put_bh(bh);
+		return NULL;
+	}
 	return bh;
 }